The present invention relates to a system for canceling interference in a polarized transmission channel caused by a frequency adjacent, or overlapping, channel of opposite polarization.
In satellite transmission systems, a plurality of transponders on the satellite are tuned to transmit a corresponding plurality of respective carrier frequencies arranged in a band of frequencies. In order to minimize interference between transponders, each transponder transmits on an antenna which is polarized. In one satellite system, e.g. the direct satellite system (DSS), the transponders transmit carriers at frequencies which are mutually different from those of all the other transponders. Each transponder transmits through an antenna which is circularly polarized. Transponders transmitting carriers adjacent in frequency are oppositely-polarized. For example if one transponder transmits at one carrier frequency and transmits through a right-hand-circularly-polarized (RHCP) antenna, then the transponder which transmits at the next adjacent carrier frequency is left-hand-circularly-polarized (LHCP). In another satellite system, two transponders transmit on a single frequency. One transponder transmits on a RHCP antenna, and the second transponder transmits on a LHCP antenna.
In an ideal implementation, using a professional grade receiving antenna, opposite polarization rejection ratios of 30 dB can be achieved. In an implementation using a consumer grade receiving antenna, the opposite polarization rejection drops to 20 dB. Furthermore, antennas configured for separately receiving oppositely polarized signals are optimized for performance at a single frequency, generally in the middle of the band of frequencies, and, therefore, operate less than optimally over a range of frequencies. In addition, atmospheric conditions may shift the polarization of the signals, further degrading the performance of the system.
Current satellite systems use modulation and coding techniques, and adjust throughput, in such a manner as to be able to function acceptably under the conditions present in the transmission system, as described above. For example, DSS systems use quaternary phase shift keying (QPSK) for modulating in-phase and quadrature carrier components with coded data at a predetermined symbol rate, in a known manner. However, it is always desirable to increase the throughput in a communications system. The increased throughput can be used to increase the number of channels which may be transmitted through the satellite, and/or to transmit extra information in existing channels to provide additional features such as higher definition images. It is further desirable in a consumer transmission system, such as the DSS system, to increase the throughput for new receivers incorporating new features, while retaining backward compatibility with existing consumer receivers.
To increase the throughput, it is necessary to increase the rejection of frequency adjacent or overlapping, oppositely polarized, signals beyond the 20 dB level. However, it is not possible to do this by increasing the power output of the transponder or by improving the performance of the transmitting antenna in the satellite, or by improving the performance of the receiving antenna, which would unacceptably increase the cost of the antenna to the consumer, nor by increasing the power output of the transponders on the satellite.
In accordance with principles of the present invention, a system for receiving a plurality of broadcast channels transmitted with different polarizations includes a first demodulator for demodulating a first channel having a first polarization to produce a first demodulated channel signal, and a second demodulator for demodulating a second channel having a second polarization, oppositely polarized with respect to the first polarization, to produce a second demodulated channel signal. An adaptive interference cancellation network, coupled to the first and second demodulators, cancels interference derived from the second channel in the first demodulated channel signal to produce a received signal.
By using electronic adaptive interference cancellation techniques to cancel interference from frequency adjacent or overlapping, oppositely polarized, broadcast channels, the rejection of those channels can be increased sufficiently that the throughput of the desired channel can be increased. For example, hierarchical quadrature amplitude modulation (QAM) of the transmitted carrier can be used to increase the bit rate of the channel, while maintaining backward compatibility. Specifically, instead of QPSK modulation, hierarchical 16 QAM or 64 QAM may be used to double or triple the number of data bits conveyed in each transmitted symbol. Current QPSK receivers can receive the first level QPSK signal transmitted on such a channel, while newer receivers, with hierarchical QAM receivers can receive the complete QAM signal, provided the opposite polarization interference from channels adjacent in frequency is sufficiently reduced using the system according to the present invention.